1. Industrial Field of Utilization
The present invention relates to cold-rolled steel sheets or hot-dip galvanized cold-rolled steel sheets for deep drawing which have excellent resistance to cold-work embrittlement or bake hardenability and more particularly to hot-dip galvanized cold-rolled steel sheets for deep drawing which have excellent deep drawability and adhesion of galvanized coating.
2. Description of Prior Art
Cold-rolled steel sheets for use for automotive parts and outer panels of electrical equipment are required to have good press-formability and good corrosion resistance in recent years.
For manufacturing cold-rolled steel sheets which can meet the above-mentioned requirements, there has been proposed a process for the individual or compound addition of carbonitride forming elements such as Ti and Nb to ultra-low carbon steel for the purpose of stabilizing C and N in the steel, thereby developing (111) texture which is advantageous for deep drawing and for galvanizing of the steel.
However, ultra-low carbon steels in which C and N in the steels are sufficiently stabilized by the carbonitride forming elements such as Ti and Nb, have a problem that cracking due to brittle fracture occurs in cold-work after press-forming. Furthermore, P-added steels have a problem that P is segregated to the grain boundary promoting brittleness of the grain boundary. This is due to the stabilization of solid-solute C in the steel, resulting in nonsegregation of C into the ferrite grain boundary and accordingly in an embrittled grain boundary. Particularly in the case of the hot-dip galvanized steel sheet, molten zinc easily intrudes this embrittled grain boundary, thus further promoting brittleness.
This hot-dip galvanized steel sheet has the problem of powdering or flaking of the galvanized coating during press-forming, that is deteriorating adhesion of the galvanized coating.
As a means of solving the aforesaid problem of the embrittlement of grain boundary, there has been attempted to melt the steels by pre-controlling the addition of Ti and Nb so that solid-solute C and N may be left in the steels. According to this method, however, even if component steels having residual solid-solute C and N can be made, this solid-solute C and N substantially acts to deteriorate the r-value and ductility of the steels, unavoidably resulting in largely lowered press-formability. That is, the press-formability and the resistance to cold-work embrittlement cannot be compatible with each other. Besides, it is technologically impossible to leave such a slight amount of solid-solute C and N in steels at the stage of steel-making.
In connection with this respect, the following proposals have been made sofar; it is, however, difficult to obtain both excellent press-formability and excellent resistance to cold-work embrittlement.
For example for the purpose of improving the resistance to cold-work embrittlement in deep drawable steel sheets there has been proposed a method of forming a carburized layer at the surface of the steel sheets by stabilizing C in steels by adding Ti and Nb and, after cold-rolling, carburizing through open-coil annealing (laid-Open Japanese Patent Application No. Sho 63-38556). In this method, however, since carburizing is applied during a prolonged period of batch annealing, a high-concentration carburized layer is formed (an average amount of C in the carburized layer: 0.02 to 0.10%) at the surface layer of the steel, and there exists a difference in ferrite grain size between the surface layer and the central layer. Furthermore, the batch annealing process is naturally not highly productive and the mechanical properties of the steel are likely to be inhomogenous in the direction of rolling and in the direction of sheet width.
There has also been proposed a method for providing only an extremely thin surface layer with a very slight amount of solid-solute C and N for the purpose of improving phosphatability (Japanese Patent Publication No. Hei 1-4233I). According to this method, however, the resistance to cold-work embrittlement is not taken into consideration. Therefore, it is impossible to perform the carburizing step required for improving the resistance to cold-work embrittlement.
Similarly, for manufacturing steel sheets for deep drawing by addition of Ti and Nb there has also been proposed a method for further carburizing after applying recrystallization annealing after cold rolling (Laid-Open Japanese Patent Application No. Hei 1-96330). This method, however, has drawbacks in that it aims mainly at providing greater strength through the precipitation of a large amount of carbides or nitrides no consideration is taken for improvement in the resistance to cold-work embrittlement; prolonged batch carburizing and nitriding are carried out, which after annealing, causes the amount of carburizing and nitriding to become excessive and nonuniform, the producibility is low and the process is complicated.
Beside the aforementioned problem as to the improvement in the resistance to cold-work embrittlement, there is an increasing demand for the provision of properties capable of increasing yield stress of steel sheets after paint baking, that is so-called bake hardenability.
In relation to the aforementioned demand, there has been proposed a method of adding a smaller amount of Ti than atomic equivalent to C for the purpose of leaving the solid-solute C (Japanese Patent Publication No. Sho 61-2732). According to this method, however, the solid-solute C and N substantially acts to deteriorate the r-value of steel even if the component steel containing the residual solid-solute C and N can be made, with the result that the press-formability is largely lowered. That is, the press-formability and the bake hardenability are substantially incompatible with each other.
Furthermore, the aforesaid process utilizing carburizing in the annealing process (Laid-Open Japanese Patent Application No. Sho 63-38556) and the process for improving the phosphatability do not take the bake hardenability into consideration, and accordingly it is impossible to improve the bake hardenability.
Furthermore, in the case of the ultra-low carbon steels stabilizing C and N sufficiently with carbonitride forming elements such as Ti and Nb, the bake hardenability is not obtainable.
Furthermore, according to the process for containing the solid-solute C, a target value, if too high, deteriorates the ageing property, and, reversely if too low, can not obtain the bake hardenability. It is very difficult to control the optimum amount of residual solid-solute carbon in the steelmaking process.